An unusual Lower East Side real-estate deal OK’d by the de Blasio administration has prompted a probe by Manhattan US Attorney Preet Bharara — the fourth investigation into the matter, according to a source.

A for-profit nursing- home operator, The Allure Group, bought the protected property in early 2015 for $28 million.

The company got the city to lift usage restrictions on the site last summer in exchange for $16 million, but Mayor de Blasio has claimed the city was hoodwinked into believing the property would continue to be operated as a nursing home.

Then, earlier this year, Allure sold the site to a group of luxury condo developers for $116 million.

While Hizzoner has threatened a suit, his administration doesn’t appear to have secured any guarantees from the firm in writing.